The present invention relates to a switch which is used in a network or a switch (referred to as a virtualization switch) which provides a device connected in a network with a virtualized storage and a computer system which includes such a switch.
As a method of enhancing reliability of data transfer between a computer and a storage system, there has been known a technique of preparing a plurality of access paths between the computer and the storage system and switching the current access path to another access path when a trouble takes place so that the computer may keep accessed with the storage system.
For example, the patent publication 1 (JP-A-11-120092) discloses the technique of an access between a computer and a storage system that controls a storage unit provided with two or more connecting ports. When a trouble takes place in one access path to the storage unit, the storage system is being accessed by the computer through another access path so that the storage system may conceal the trouble from the computer.
Moreover, the patent publication 2 (the Official Gazette of JP-A-10-021016) discloses the technique of switching an access path between a storage system and a computer. The storage system detects a trouble occurring in an access path to a storage unit included in the storage system itself and gives back an error response for indicating use of another access path to the computer so that the computer may reissue a request for accessing another access path (referred to as a command), for switching the access path.
In the patent publication 1, the storage system may conceal a trouble occurring in the access path from the computer. To conceal the trouble, it is essential to locate in the storage system a storage unit like a buffer that temporarily saves data being transferred. This is because the device that tries to conceal the trouble is required to retry transfer of data being executed when the trouble has occurred through another access path (referred to as an alternate path).
Herein, consider the storage system like a RAID as the device to which the technique disclosed in the patent publication 1 is applied. In this case, the storage system often includes a cache memory for the purpose of improving the performance. This cache memory may be thus used as a buffer that saves a retry command and data to be transferred.
Today, however, a storage area network (often abbreviated as an SAN) of connecting a computer with a storage system has risen in this technical field. It is thus requested to supply the device composing the SAN and for relaying data (referred to as a switch) with a capability of switching an access path. In particular, in a case that the switch virtualizes a storage area of the storage system and provides the computer with one or more virtual storage systems, the switch is required to manage an access path between the switch itself and the storage system.
In this case, the application of the technique disclosed in the patent publication 1 to the switch burdens the switch with an extra memory, which results in making the overall system more costly.
In the technique disclosed in the patent publication 2, on the other hand, for realizing the switch of an access path, the storage system notices a trouble occurring in an access path to the computer and the computer reissues a command for an alternate path. Hence, the present system is required to add a capability of switching an access path to the computer. For this requirement, it is necessary to install software and the like for switching an access path in all computers connected in the network, which also results in making the overall system more costly.